1. Field of the Invention
The present invention relates to a frequency modulator that includes a voltage controlled oscillator for frequency-modulating an input signal and outputting a frequency-modulated signal, and a method for adjusting the gain thereof.
2. Description of the Background Art
Communication apparatuses such as mobile phones and wireless LAN devices are required to operate with low power consumption while ensuring the accuracy of an output signal. Thus, such communication apparatuses need to be equipped with a frequency modulator that is capable of outputting, as an output signal, an RF phase-modulated signal with low distortion. The following will describe a conventional frequency modulator.
As a conventional frequency modulator, there is known a frequency modulator 100 that is disclosed in International Publication WO 96/33553. FIG. 19 shows a configuration of the conventional frequency modulator 100. In the frequency modulator 100, an inputted modulation signal is band-limited to a specific frequency range by the band limiter 101, and inputted to a buffer 102. The modulation signal inputted to the buffer 102 is inputted to a high frequency circuit 103 for modulating the modulation signal with a high frequency component and a low frequency circuit 104 for modulating the modulation signal with a low frequency component. A voltage controlled oscillator (VCO) 105 adds the modulation signals outputted from the high frequency circuit 103 and the low frequency circuit 104, and outputs an output signal having a frequency according to the voltages of the added modulation signals.
In the low frequency circuit 104, a reference frequency signal is generated by a temperature compensated crystal oscillator (TCXO) 107. The reference frequency signal is divided down to a lower frequency by a frequency counter 108. A phase detector 109 outputs a voltage signal proportional to the phase difference between the reference frequency of the reference frequency signal outputted from the frequency counter 108 and the output signal of the VCO 105 that is fed back by a frequency counter 111. A loop filter 110 filters out all frequency components of the voltage signal, which is outputted from the phase detector 109, below the cut-off frequency thereof, and outputs the filtered signal to the VCO 105.
The high frequency circuit 103 includes a buffer 113 and a variable gain amplifier 114. The variable gain amplifier 114 adjusts the magnitude of an output signal from the buffer 113, and outputs the output signal to the VCO 105. Thus, the gain between the modulation signal inputted to the VCO 105 through the low frequency circuit 104 and the modulation signal inputted to the VCO 105 through the high frequency circuit 103 is balanced.
When the gain of the VCO 105 is great, the gain of the VCO 105 can be kept constant by reducing the gain of the variable gain amplifier 114. Here, it is assumed that the variable gain amplifier 114 adjusts the gain of a modulation signal during a digital signal process and the effective number of bits is 10 bits. When the gain of such a variable gain amplifier 114 is reduced to ½ and further to ¼, the effective number of bits also decreases to 9 bits and further to 8 bits. Thus, a sufficient effective number of bits of the variable gain amplifier 114 cannot be ensured, for example, only 8 bits are useable even though there is originally a digital region of 10 bits. This results in a problem that quantization noise increases.